1. Field of the Invention
The invention relates to a system for transmitting energy to an apparatus lowered inside a well or drill hole.
The system applies in particular to supplying with energy to seismic sources operating in wells down to relatively great depths.
Many seismic prospection methods comprise the use of seismic sources lowered into wells or drill holes and activated successively at a plurality of positions at different depths. The waves emitted, are received by receivers disposed in other wells or drill holes, which allows a high power of resolution to be obtained, or else they are disposed on the surface so as to restore oblique seismic profiles. The energy efficiency of seismic sources in wells is generally much better than that of sources operating on the surface, because they emit the seismic waves under the weathered layer whose propagation characteristics are unfavorable.
2. Description of the Prior Art
A known method for supplying the energy required for the operation of seismic sources operating at relatively small depths consists in connecting them by piping to a hydraulic device installed at the top of the well. The pressurized hydraulic fluid which is delivered to them is used for producing vibrations or pulses. Such a method, which is described for example in the U.S. Pat. Nos. 3,221,833 an 3,718,205, is unsuitable when the operating depth becomes great because of the difficulty in providing hydraulic connections in wells which are often of small dimensions.
In another known method, potential energy is accumulated at a point in a well, at a relatively small depth, by raising, from a surface installation and by means of cables, a weight which may slide about a guide tube. At the base thereof is fixed a target element. With intermittent anchorage means the target element may be immobilized, and the associated guide tube, at a chosen depth. The accumulated energy is released by the weight falling against the target element, which generates seismic waves. Such a method which is described for example in the European patent application EP No. 136,236, is suitable for use down to relatively small depths in wells which are uncongested, relatively vertical and not filled with liquid, because the weight is not isolated from the well and because it is reset by pulling on cables.
Another system more compatible with supply of seismic sources operating at great depths includes a surface installed electric generator and electric conductors for connecting them thereto. These conductors are very often gathered together with data transmission conductors in a single multiple function electro-carrying cable. Such a system is used, for example, for supplying with electric energy pulsed seismic sources such as those described in the published, French patent applications Nos. 2,555,761, 2,558,601 and 2,563,919, where the energy required for generating the pulses is supplied by a hydraulic pump driven by an electric motor fed from the surface by the electro - carrying cable.
The drawback of electro-carrying cables is their relatively low current flow. The electric power which they may transport becomes quickly insufficient for resetting seismic sources or causing them to operate at great depths, because of the very high hydrostatic pressure which reigns there.
A method is also known for generating seismic waves at the bottom of a well being drilled, which consists essentially in inserting a telescopic part between the drill-pipe string and the drilling tool, and intermittently releasing the tension exerted from the surface so that this telescopic part retracts.
Such driving in accumulates hydraulic energy. With a certain delay it is transmitted to the drilling tool which strikes against the bottom of the drill hole. This method, which is described in the US patent No. 2,898.084, is adapted for use exclusively at the bottom of a well.